Head mounted display device

ABSTRACT

A head mounted display device comprises a display which is imaged on the eye at an adjustable angle of incidence. On the basis of information about a position of the head of the wearer, the angle of incidence is controlled such that, upon movement of the head, the angle of incidence of the image moves along with that of the outside world.

The invention relates to a head mounted display device. A known form ofa head mounted display device is a helmet with a display attachedthereto. As the display is mounted on the helmet, the display movesalong with the head of the wearer. As a result, it is possible, with ahead mounted display, contrary to a free display such as a conventionalmonitor, to keep the image of the display constantly in front of theeyes of the wearer, independently of the position of the head of thewearer.

A known application of head mounted display devices is “virtualreality”. In that application, a computer adjusts the contents of theimage on the display to the position of the helmet, and hence to thedirection in which the head of the wearer is turned. The computer doesthis in a manner such that the contents of the image correspond to whatcan be seen in that direction in a virtual world. The wearer thus getsthe impression that he or she can look around him/herself in thisvirtual world.

This technique can also be used to make a virtual document visible to awearer in a fixed direction. However, it has appeared that reading suchdocuments proceeds considerably slower and has a strongly tiring effect.Such a head mounted display device is known from WO 97/20244. Thisdevice shifts the image on the display in response to orientation of thedevice. No mention is made of shifting the image of the display, ratherthan the image on the display.

An article by Fisher, titled “Variable acuity remote viewing system(VARVS), published in the proceedings of the National Aerospace andElectronics conference (NAECON) Dayton, May 16–18 1977, by the IEEE NewYork Vol 3 conf 30 on pages 1172–1179, describes the presentation of ahigh resolution image part to the fovea of the human eye. As is wellknown the fovea is area at the back of the eye where the eye has highestresolution. Normally, the orientation of the eyeball determines whichpart of the surrounding world is imaged onto the fovea. The projectionsystem of Fisher has a high resolution part and Fisher strives toproject this high resolution part from a direction that makes theeyeball image the high resolution part onto the fovea.

Most embodiments of Fisher do not use a head mounted screen and aretherefore irrelevant for the present application. Fisher mentions onlyone embodiment that uses a head mounted display (FIG. 15). Thisembodiment apparently works under the assumption that the wearer willkeep the eye direction fixed relative to the head during head movements,so that the same part of the image on the head mounted display remainsimaged on the fovea. No mention is made of moving an apparent directionrelative to the head mounted display along which the image of the screenis projected to the eye of the wearer.

One object of the invention is to provide a head mounted display devicethat has a less tiring effect.

The invention provides a head mounted display device provided with:

-   -   a supporting frame for fastening the head mounted display device        to the head of a wearer;    -   a display for generating an image;    -   imaging optics for imaging the image to an eye of the wearer;    -   movement means for changing at least one angle of incidence at        which the display is displayed via the eye;        -   a position detector for measuring information about a            position of the supporting frame; and        -   a controlling coupling from the position detector to the            movement means, arranged such that the movement means change            the angle of incidence relative to the supporting frame in            accordance with changes in the position of the supporting            frame, so that the angle of incidence of the image falls on            the eye while moving along with the outside world. The angle            of incidence where the display is imaged can for instance be            moved by moving the display relative to the supporting            frame, or by adjusting the imaging optics.

The invention is based on the observation that the human eye tends tocompensate in advance the effect of movements of the head. With a fixedeye position, a movement of the head would cause a displacement of theimage of the environment on the retina. However, the eye involuntarilycompensates for the head movement by making movements with an oppositeeffect. As a result, displacements resulting from head movements arenormally compensated.

However, in a head mounted display device in which the display movesalong with the head, the displacement is absent, or is suggested bychange of the contents of the image. This latter occurs for instance inso-called virtual environments where a computer adjusts the image to theviewing direction of the user. Disadvantages of this method are that acomputer is required and that the image compensation is accompanied by adelay and sometimes by image artifacts. Consequently still, a changingimage is formed on the retina, which, particularly when readingdocuments, has a strongly tiring effect.

The invention is based on the insight that this effect can be preventedby providing that the angle of incidence at which the display is imagedon the eye relative to the outside world is fixed, independently ofsmall head movements. This is realized with relatively smalldisplacements of the display or small adjustments to the imaging optics(up to an angle of typically 5 degrees). Contrary to virtual realityapplications, the content of the image on the display does not need tobe adjusted.

The invention is particularly intended for “reading” documents, maps,etc., for which a focused action of the eye is required, generally in alimited image field. Naturally, displacing the display or adjusting theimaging optics is not a practically useful equivalent to the change ofthe contents of the image such as are necessary for compensating largerhead movements (for instance through ninety degrees), for instance invirtual reality applications. This is because the display, bydisplacement thereof, would then be imaged only peripherally in thefield of vision, or would even fall outside the field of vision. Insofaras there is a need to follow these head movements too, this willtherefore have to be done by changing the contents of the image.

In principle, all sorts of head movements can be compensated, such ashead movements which cause the image to move horizontally or vertically,and head movements which cause the image to rotate. Preferably, at leastvertical movements are compensated. In practice, these are the mostdisturbing movements. In one embodiment, the device only compensatesthese movements. Thus, at low costs, the most annoying consequence ofhead movements is removed.

In one embodiment of the device, rapid changes in the angle of incidenceare compensated, without compensating an average change of the angle ofincidence over a longer period of time. Hence, the wearer of the deviceis not bound to a fixed viewing direction.

In one embodiment of the head mounted display device according to theinvention, the movement of the angle of incidence is realized bydisplacing the display relative to the supporting frame. As analternative, the imaging optics can be adjusted, for instance by using aprism with adjustable angle, or by shifting lenses in a manner knownfrom movement compensating binoculars and/or cameras. However, movingthe display is preferred, for one thing because changing decenteringaberrations of the imaging optics are thereby avoided.

These and other advantages and objectives of the head mounted displaydevice according to the invention will be further described withreference to the following figures.

FIG. 1 shows a head mounted display device;

FIG. 2 shows a display movement mechanism.

FIG. 1 shows a diagrammatic overview of a head mounted display device onthe head of a wearer 18. The device contains a supporting frame 10, withwhich the device is fastened to the head 18, a display 12 movablyattached on a bed 17, imaging optics 14, 15 and a position detector 16.The embodiment shown of the imaging optics 14, 15 is not essential tothe invention. By way of example, a lens 14 and a mirror 15 are shown,which guide a light beam 13 from the display 12 to an eye of the wearer.The position detector 16 is coupled to the movement mechanism of the bed17.

The image represented on the display comes, for instance, from acomputer (not shown), and contains, for instance, an image of a documentas produced by a text editor. However, the invention is not limitedthereto: the image can also come, for instance, from video equipment(not shown), or be generated with computer graphics techniques. Suchimage sources need not form part of the head mounted display device andcan also be placed detached from the supporting frame 10.

In principle, the head mounted display can contain two displays, one foreach eye. Also, with the aid of an image splitter (for instance a semitransparent mirror) the same display can be imaged on both eyes.Further, an embodiment is possible in which the device images an imageon one eye only.

FIG. 2 shows a display movement mechanism for movement of a displayalong one direction. The display 12, the position detector 16, a sensor20, a controller 22, an actuator 24 and a guide 26 are shown. The guide26 is fixed relative to the supporting frame 10 (not shown in FIG. 2).The actuator 24 is coupled to the display 12 for moving it back andforth relative to the guide 26. The sensor 20 is provided for measuringthe displacement (i.e. for instance speed or acceleration) of thedisplay 12 relative to the guide 26. The position detector 16 isarranged for measuring an angular displacement (for instance an angularvelocity or angular acceleration) of the supporting frame relative tothe outside world (which does not co-accelerate with the headmovements). All sorts of angle detectors or angular accelerationdetectors known per se are possible, such as detectors based on agyroscope, or detectors for the earth's magnetic field. The measuredangular displacements are fed from the sensor 20 and the positiondetector 16 to the controller 22, which, in turn, controls the actuator24.

By way of illustration, the invention will be described for compensationof head movements whereby the head 18 moves in a direction such that theimage moves vertically up and down.

Movements of the head causing the image to move in vertical directioncause the principal problems in the use of a head mounted display. Forthat reason, preferably, at least one movement mechanism is included forcompensating movements in vertical direction. However, without departingfrom the invention, preferably also movement mechanisms are included tocompensate movements in other directions, such as movements whereby theimage moves in horizontal direction and/or whereby the image rotates.Further, also, the effects of translations of the head can becompensated, in case the impression is to be created that the image isat a particular distance (for instance a normal reading distance) fromthe head.

In operation, an image on the display 12 is imaged by the imaging optics14, 15 via the eye of the wearer 18. By way of example, simple opticsare shown. The optics include a lens 14, which forms an image of thedisplay such that the eye, when focusing at a particular distance, forinstance, infinity, sees the image sharply. Further, the optics includea mirror 15 which mirrors the image into the field of vision of the eyesof the wearer 18. Preferably, the mirror 15 is slightly transparent, sothat the wearer 18 can also observe the surroundings beyond the mirrorto some extent. In practice, more complicated optics can be used,depending on the need to mount the display 12 at a different location onthe supporting frame 10, or to correct for optical imaging errors.

As the display 12 and the optics 14, 15 are fitted on the supportingframe 10, in principle, the image moves along with the movements of thehead 18. The movement mechanism compensates this movement in at leastone direction.

The position detector 16 detects angular accelerations of the supportingframe 10 in vertical direction, i.e. in the direction about an axisparallel to an axis between the ears (the “nodding axis”). In responseto the detected accelerations, the controller 22 adjusts the position ofthe display 12.

In principle, there is a fixed factor between the displacement distanceof the display and the change of the angle at which a point on thedisplay is observed. This factor depends on the imaging optics. Thecontroller 22 makes a comparison corrected for this factor between theangular acceleration of the supporting frame 10 as detected withposition detector 16, and the acceleration of the display 12 as detectedby sensor 20, and controls the movement of actuator 24 so that theseaccelerations, corrected for the factor mentioned, substantiallycorrespond.

Consequently, the image of the display 12 remains in a fixed position inat least one direction (for instance the vertical direction) relative tothe outside world. There is a constant angle between a horizon and thedirection in which a light beam 13 falls on the eye from a fixed pointon the display 12. Viewed from the wearer 18, the display therefore hasa fixed position relative to the horizon. Hence, viewed from the wearer,the display moves or vibrates along with the horizon upon headmovements. Through the involuntary compensation carried out by the eyefor these head movements, the image of the display remains stablyobservable.

It is not necessary that the movement of the display compensates toogreat head movements, or that the average of the head movements over alonger period of time (i.e. the low frequency components of the headmovement) is compensated. Major head movements occur less often thanminor head movements. Accordingly, upon major movements, it is lessimportant to be able to continue reading documents. In fact, for readingdocuments, it is preferable that the image is observed at a position onaverage straight in front of the head. The mechanism of FIG. 2 isarranged such that only higher frequency components of the head movementare compensated, in that control takes place based on accelerations.

In the embodiment shown, the difference in measured angular accelerationresulting from head movement and the measured acceleration of thedisplay movement are controlled to zero. Consequently, the average ofthe angle at which the head is present will not be controlled to zeroand, for rapid movements (for instance with movement frequencies above1/3 Hertz), a relatively small movement stroke of the display 12 willsuffice, for instance corresponding to head movements of up to four orup to eight degrees. A similar effect is obtained by controlling to zerothe difference in angular velocity or a combination of angular velocityand acceleration. It is even possible, however, to control angulardifferences to zero, albeit that this has the disadvantageous effectthat, upon displacement of the head such that, on average, it looks in adifferent direction, the document is no longer averagely imaged in themost convenient viewing direction.

The combination of the controller 22 and actuator 24 does not need toreact infinitely rapidly. A bandwidth for following movements with afrequency up to approximately five Hertz has proven quite sufficient. Itis noted that this is much lower than the image refreshing frequencyrequired for artifact-free observation of images. The reason for this isthat only compensation of head movement is necessary that are not toorapid because of the relatively large mass of the head.

In the embodiment shown, a feedback loop is utilized for setting theposition acceleration of the display. The loop gain determines theminimal frequency of the angular changes which are controlled to zero.Naturally, the invention is not limited to a feedback loop: also a feedforward system can be used with a comparable frequency characteristic,with which the detected angular changes directly control the positionchanges of the display. This latter has the advantage that the speed ofresponse of the control becomes higher, as a result of which the displaycan be observed even more agreeably.

The embodiment shown shows provisions for displacement in one direction,namely for compensation of rotations of the head about an axis “from earto ear”. Naturally, without departing from the invention, comparableprovisions can be provided for moving the display 12 in other directionsand/or rotating it about an axis perpendicular to the plane of thedisplay 12. Thus, displacements as a result of head movements other thanabout the axis from ear to ear can be compensated. To this end, theangle displacement sensor 16 is oriented for detecting rotations aboutthe respective axis: an axis parallel to the neck of the user or an axisin the viewing direction of the wearer when he or she looks straightahead.

In a further embodiment, provisions can be made for compensating headmovements in more than one direction simultaneously. In this embodiment,on the supporting frame 10, several angular displacement sensors areincluded, for measuring angular displacements about the rotational axesinvolved. The measured angular displacements about these axes are used,respectively, to control the displacement of the display 12, forinstance in horizontal and vertical direction and with pivotal movementof the display 12 about an axis perpendicular to the surface of thedisplay, so that the change in angle of incidence and pivoting movementsof the image as a result of rotations of the head are compensated.

Further, in one embodiment, also the effect of translations of the headcan be compensated (and so not only of rotations). The effect oftranslations need not be compensated as long as the impression is to becreated that the image of the display 12 is at infinity. But if theimpression is to be created that the image is at a particular distance(for instance normal reading distance), then preferably also the effectof translations is compensated. This is for instance realized by alsoincluding translation sensors, and providing a calculating unit (forinstance a microprocessor) which, on the basis of the measured rotationsand translations of the head, calculates with elementary geometricformulas what would have to be the change of the angle or angles ofincidence of a light beam of an object at a particular distance from thehead. This calculated angle or these calculated angles of incidence willthen be used to control the position and/or rotation of the display 12in the manner as described hereinabove.

Although, in the embodiment shown, an angle sensor is utilized fordetecting angular displacements of the head, it is also possible, fordetecting these angular displacements, to use sensors for position (xyz)displacement, in combination with a calculating unit, such as amicroprocessor. With the aid of elementary geometrical relations, such acalculating unit can calculate an angular displacement from thedifference in displacement measured by two sensors for displacement inthe same direction at different positions on the head (this concernsangular rotations about a normal to the plane through the connectingline between the sensors and the direction in which the displacementsare detected). In the case that both a translation and a rotation of thehead are to be measured, this can be done with such a pair of sensors.

Although the embodiment shown, in which the display 12 is moved relativeto the supporting frame 10, is preferred, the movement of the image canalso be realized in different manners, for instance by movement of theimaging optics, as by a rotating movement of the mirror, tilting orshifting of lenses, etc. Also, a prism with an adjustable wedge anglecould be included in the optical path from the display 12 to the eye.Such prisms are known per se (Vary-Angle prism) and are realized, forinstance, by means of two transparent plates, connected with a bellows,with a transparent liquid between the plates. However, it is preferredto move the display 12, because this obviates decentering aberrations inthe imaging optics and because the mass of a miniature display issignificantly smaller than that of an optic lens, so that a smaller andlighter mechanism can suffice.

1. A self actuating head mounted display device, comprising: asupporting frame adapted to be worn on the head of a user and to therebyattach the head mounted display device to a head of a wearer; a displayfor generating an image, which display is operatively connected with thesupporting frame; imaging optics for imaging the image via an eye of thewearer, which imaging optics are operatively connected with thesupporting frame; and a display movement mechanism operatively connectedbetween the supporting frame and the display whereby at least one angleof incidence at which the display is imaged via the eye is changed bythe display movement mechanism; wherein the display movement mechanismincludes a position detector that measures information about a positionof the supporting frame, an actuator that changes the angle of incidenceat which the display is imaged via the eye, and a controlling couplingoperatively connected between the position detector and the actuator andarranged such that the display movement mechanism changes the angle ofincidence relative to the supporting frame in accordance with changes inthe position of the supporting frame, so that the angle of incidence ofthe image, moving along with the outside world, falls on the eye.
 2. Aself actuating head mounted display device according to claim 1, whereinthe coupling is arranged to change the angle of incidence in response tochanges in the angle of incidence with respect to the outside world in ahigher frequency range, virtually without changing the angle ofincidence in response to changes in a lower frequency range, so that anaverage of the angle of incidence in the longer term does not move alongwith the outside world.
 3. A self actuating head mounted display deviceaccording to claim 2, wherein the head mounted display device isarranged to change the angle of incidence in only one direction, adirection corresponding to rotation about an axis from ear to ear of theuser.
 4. A self actuating head mounted display device according to claim2, wherein the display movement mechanism is arranged for changing twoangles of incidence relative to a first and second plane, respectively,which are not mutually parallel; the position detector is arranged fortaking in information about angles of the supporting frame relative tothe first and second plane; the controlling coupling is arranged suchthat the display movement mechanism changes the angles of incidencerelative to the supporting frame in accordance with changes in theposition of the supporting frame, so that the image falls on the eye atfixed angles relative to an outside world, virtually independently ofthe changes of the position of the supporting frame.
 5. A self actuatinghead mounted display device according to claim 2, wherein the displaymovement mechanism is arranged for changing a rotation angle with whichan axis in the display is imaged relative to an axis perpendicular to aviewing direction of the eye of the wearer; the position detector isarranged for taking in information about a rotation angle of thesupporting frame relative to the viewing direction; the controllingcoupling is arranged such that the movement means change the rotationangle relative to the supporting frame in accordance with changes in therotation of the supporting frame, so that the image falls on the eyeunder fixed rotation relative to an outside world, virtuallyindependently of the changes of the position of the supporting frame. 6.A self actuating head mounted display device according to claim 1,wherein the head mounted display device is arranged to change the angleof incidence in only one direction, a direction corresponding torotation about an axis from ear to ear of the user.
 7. A self actuatinghead mounted display device according to claim 6, wherein the displaymovement mechanism is arranged for changing two angles of incidencerelative to a first and second plane, respectively, which are notmutually parallel; the position detector is arranged for taking ininformation about angles of the supporting frame relative to the firstand second plane; the controlling coupling is arranged such that thedisplay movement mechanism changes the angles of incidence relative tothe supporting frame in accordance with changes in the position of thesupporting frame, so that the image falls on the eye at fixed anglesrelative to an outside world, virtually independently of the changes ofthe position of the supporting frame.
 8. A self actuating head mounteddisplay device according to claim 6, wherein the display movementmechanism is arranged for changing a rotation angle with which an axisin the display is imaged relative to an axis perpendicular to a viewingdirection of the eye of the wearer; the position detector is arrangedfor taking in information about a rotation angle of the supporting framerelative to the viewing direction; the controlling coupling is arrangedsuch that the movement means change the rotation angle relative to thesupporting frame in accordance with changes in the rotation of thesupporting frame, so that the image falls on the eye under fixedrotation relative to an outside world, virtually independently of thechanges of the position of the supporting frame.
 9. A self actuatinghead mounted display device according to claim 1, wherein the displaymovement mechanism is arranged for changing two angles of incidencerelative to a first and second plane, respectively, which are notmutually parallel; the position detector is arranged for taking ininformation about angles of the supporting frame relative to the firstand second plane; the controlling coupling is arranged such that thedisplay movement mechanism changes the angles of incidence relative tothe supporting frame in accordance with changes in the position of thesupporting frame, so that the image falls on the eye at fixed anglesrelative to an outside world, virtually independently of the changes ofthe position of the supporting frame.
 10. A self actuating head mounteddisplay device according to claim 9, wherein the display movementmechanism is arranged for changing a rotation angle with which an axisin the display is imaged relative to an axis perpendicular to a viewingdirection of the eye of the wearer; the position detector is arrangedfor taking in information about a rotation angle of the supporting framerelative to the viewing direction; the controlling coupling is arrangedsuch that the movement means change the rotation angle relative to thesupporting frame in accordance with changes in the rotation of thesupporting frame, so that the image falls on the eye under fixedrotation relative to an outside world, virtually independently of thechanges of the position of the supporting frame.
 11. A self actuatinghead mounted display device according to claim 1, wherein the displaymovement mechanism is arranged for changing a rotation angle with whichan axis in the display is imaged relative to an axis perpendicular to aviewing direction of the eye of the wearer; the position detector isarranged for taking in information about a rotation angle of thesupporting frame relative to the viewing direction; the controllingcoupling is arranged such that the movement means change the rotationangle relative to the supporting frame in accordance with changes in therotation of the supporting frame, so that the image falls on the eyeunder fixed rotation relative to an outside world, virtuallyindependently of the changes of the position of the supporting frame.